The principal objective of this research is to understand the molecular mechanisms of mammalian DNA transcriptional control in normal cell and how alterations in such control may lead to malignancy. The major approaches are through 1) direct biochemical and biophysical analysis of defined, highly repetitive DNA sequences and their associated nucleoproteins by specific excision of the sequences from intact mammalian nuclei with restriction enzymes, 2) long-range DNA sequencing of the repeat units, 3) studies of DNA methylation patterns of the repeat sequences and 4) analysis of the nucleoproteins, including the histone and nonhistone proteins associated with the specific sequences. By focusing on simply organized and abundant DNA sequences in mammalian nuclei, unique opportunities are presented for understanding how specific nucleoproteins may interact with defined DNA sequences and thereby regulate gene expression. Also, alterations in highly repetitive mammalian DNA sequences and DNA methylation have been strongly implicated in the processes of neoplastic transformation. The studies will be carried out in the context of present knowledge of chromatin subunit structure to expand that knowledge and to define the macromolecular configurations or chromatin packaging that may determine an important mode of transcriptional inactivation in the mammalian nucleus. Studies are also continuing to determine 1) the properties of high molecular weight homopolymer-like sequences in mammalian nuclei with special reference to their packaging in chromatin subunits, 2) the possible roles and mechanisms of highly repetitive DNAs in mediating massive chromosome rearrangements in mammalian cells and 3) the modes of DNA replication of the highly repetitive DNA sequences.